Summary:
Property Geology
The Sangdong Project is situated on the southern limb of the east-west orientated Triassic age Baekunsan (Hambaek) Syncline. Cambro-Ordovician limestone, shale, and quartzite of the Chosun System unconformably overlie the Pre-Cambrian schist and the gneiss of the Yulri Series.
The Property area is underlain by metasedimentary rocks belonging to the Yangdok and Great Limestone Series that are situated on the south limb of the asymmetrical Hambaek syncline that plunges gently to the southeast; with a strata strike of approximately 110° and dip to the north-northeast at 20° to 30°.
Mineralisation
The Sangdong deposit is a stratabound W-Mo ore deposit. The tungsten mineralisation is contained in several tabular, bedding conformable skarns in the Myobong Shale; these skarns have been interpreted as comprising carbonate-bearing horizons that were altered and mineralised and derived from the metasomatism produced by hydrothermal fluids ascending from the underlying Sangdong Granite. Kang et al (2022) referred to a consistent association of the Sangdong granite with W-Mo mineralization. This granitic rock shows S-type characteristics and the Rb and Sr contents fall in the domain of the giant reduced W skarn deposits. Moon (1987) referred to W content up to 62 ppm and Mo content up to 20 ppm, for the Sangdong Granite.
The skarn formation can be subdivided into prograde (stages I and II) and retrograde (stages I and II) skarn stages, based on their mineral assemblages, and followed by the vein stages. The mineral assemblage associated with the prograde stage contains clinopyroxene, garnet and wollastonite, with accessory mineral phases such as scheelite, fluorite and titanite and occurs in the margins of the skarn orebodies. The mineral assemblage related with the retrograde stage is located between the margins and the center of the orebodies and their mineral assemblage comprises amphibole and biotite with minor scheelite, fluorite, apatite, epidote, vesuvianite, magnetite and disseminated sulphides (Kang et al., 2022).
From uppermost to lowermost, these skarn horizons are termed the Hangingwall, Main, and Footwall horizons. Calc-silicate layers from 0.50-1.0m in thickness have developed on the upper and lower contacts of the Main and Footwall horizons.
The Hangingwall horizon is located near the upper contact of the Myobong Shale and varies in thickness from approximately 5.0 to 30.0m because of the irregular boundary of the shale with the overlying Pungchon Limestone. The maximum thickness is over 100m and occurs near the center of the deposit (Moon, 1991). This zone has a strike length of about 600m and a down-dip extent of about 800m. Above the most highly-altered portion of the Main horizon, the Hangingwall horizon is not tabular, but extends steeply and irregularly into the overlying limestone. The Hangingwall horizon contains diopside, garnet, fluorite, zoisite, quartz, hornblende, wollastonite and up to 50% calcite and although there is some zonal variation in mineral assemblages (diopside-, hornblende- and quartz- rich zones) the zonation is not as well-developed as in the underlying Main horizon. The tungsten values show some zonation and decrease in value up-dip. The base of the Hangingwall horizon is approximately 14m above the upper contact of the Main horizon.
The Main horizon strikes about 100° and dips northerly between 15° and 30°. The strike length is in excess of 1,300m and thickness varies from 5.0 – 6.0m. Hydrothermal alteration (skarnification) within the Main horizon forms three concentric, roughly circular zones. A central quartz-rich zone, consisting of muscovite, biotite, quartz and minor chlorite, is about 350m in diameter, plunging down the Main horizon at N05°W and is coincident with the higher tungsten grade portion of the deposit. The central zone is succeeded outward by a hornblende-rich zone containing diopside, hornblende or tremolite, chlorite, fluorite and calcite. A diopside-rich zone occurs both horizontally beyond and stratigraphically above the hornblende-rich zone and contains garnet, diopside, quartz, fluorite, zoisite and plagioclase. The diopside zone is typically poorly-mineralised. Boundaries between these zones are diffuse and transitional.
The Footwall horizons comprise multiple layers: Footwall Zone 1 (F1) occurs 1m below the Main horizon and is approximately 2m thick; Footwall Zones 2 and 3 (F2, F3) are situated approximately 35.0 to 40.0m below the Main horizon typically 3-4m thick. Further Footwall Zones have been identified below F3 and are collectively referred to as F4 and F5. Lateral dimensions of these horizons and the zonal distribution of calc-silicate minerals in them are similar to those of the Main horizon. The F1 zone has sometimes been mined with extraction of the Main Zone. Some parts of F2 and F3 have been mined in the upper section of the mine (with the exception of the section between Taebaek and -1 mine levels).
Sangdong East (East WO3 Orebody) is located about 1km to the east from the Main deposit. It is essentially an extension of the main mine area, and stratigraphy and lithologies are similar. In contrast, however, the Hangingwall (Upper) horizon, Main horizon, and Footwall (Lower) horizons are thinner, and have a lower frequency of quartz veins. The constituent minerals are pyroxene and garnet with accessory plagioclase, quartz, apatite, hornblende and wollastonite.
In the Sangdong West area (West WO3 Orebody), mineralisation and stratigraphy are similar to the main Sangdong Mine, but skarn horizons are thinner, and there is a lower frequency of quartz veining than in the Main horizon. The vein width and grade of tungsten mineralisation do, however, increase northward to the Hwajeolchi area. Scheelite and molybdenite occur together in quartz veins, which, according to Lee (2001), is rare in the Myobong Shale.
Deposit types
Tungsten Skarns
The Property contains a tungsten skarn deposit; skarns are contact metasomatic deposits, exploited for tungsten, with accessory molybdenum, copper, tin and zinc. They typically form in continental marginal settings, associated with syn-orogenic plutons that intrude and metamorphose deeply buried sequences of carbonate-shale sedimentary sequences. Skarn mineralisation is typically hosted by pure and impure limestones, calcareous to carbonaceous pelites.
Due to their contact metamorphic nature, mineralisation has a close spatial association with calcalkaline granitic intrusives (tonalite, granodiorite, quartz monzonite and granites). Skarn deposits form stratiform, tabular and lens-like deposits, which can be continuous for hundreds of metres along intrusive contacts.
Granite Related Molybdenum
Due to the paucity of information about the molybdenum-mineralised system beneath the Sangdong underground workings, it is difficult to characterise a model for this mineralisation. However, important molybdenum mineralisation falls into two classes: porphyry-type and granite-related molybdenumtungsten-tin systems. There is some overlap between the two.
The tungsten mineralisation of the Sangdong Deposit is contained within a series of tabular skarn horizons within the Myobong Slate. Calcium carbonate horizons within the slate have undergone metasomatic replacement to mineralised skarn by hydrothermal fluids. The source of these fluids is thought to be the underlying Sangdong Granite. K-Ar age determination of phyllites within the Myobong Formation are consistent with the age of the granite below.